Nanomaterials have become important in recent years because of the markedly different electrical, optical and structural properties of these as compared to those in the conventional bulk form. For example, semiconductor nanocrystals exhibit a continuous shift of the absorption energy to higher energies due to quantum confinement. Further, it is an observed fact that morphology of nanomaterials like spheres, rods, needles, wires and tube play an important role in deciding application of these. This makes study of nano-structured materials one of the frontier areas of research in synthesis chemistry and condensed matter physics. The enormous global interest in the field is also due to the fabrication of devices using nanomaterials with special and advanced features. Nanotechnology being an emerging and pioneer area, there is a lot of scope for development of new techniques and processes for preparation of nanomaterials in different shapes.
The development of nanomaterials for devices and varied applications will depend on basic understanding of the mechanism of enhancement in properties with respect to morphology and changes in crystal structure of these down to quantum level. These developments are demanding refinement of existing and a search for newer preparation processes of nanomaterials with an emphasis on control of particle shape and dimensions. A number of synthesis processes for nanomaterials such as chemistry by microwaves, mechanochemistry, self-assembly, lithography, template and membrane based synthesis have been developed and employed by scientists, chemists and engineers the world over, but the researcher will always be on look out to produce nanomaterials with a cheaper, cleaner, reliable and industrially feasible techniques and be able to exercise precise control over important characteristic of particle aspect ratio and structure. Realization of these challenges is possible only with control of thermodynamics and kinetics of reaction, nucleation, growth, aging etc. in a process.
The valence state of multivalent elements in their compounds singly or in combination with one or more compounds has profound effect on their properties and hence the applications. Different colour of ferrous and ferric salt solution is sighted as an example. A process for preparation must take care of valence state along with crystallization of parent as well as dopant atoms in the mass as well as surface of nanomaterials since surface to volume ratio of these materials is very high.
A number of patents have appeared on preparation of nanomaterials with different processes. Matson et al filed U.S. Pat. No. 5,238,671 in August 1993 for preparation of nanomaterials by chemical reactions involving reverse micelle/microemulsion systems. In this a microemulsion of a polar fluid e.g. an aqueous fluid is made in non-polar fluid in supercritical state. Reactants are introduced into the micelles via non-polar fluid, which is a continuous phase. Gallagher et al patented (U.S. Pat. No. 5,525,377 June 1996) a method of manufacturing encapsulated doped particles of size <100° A. Nanomaterial is precipitated with a coat of a surfactant by first encapsulating an organometallic compound containing one element of host material and subsequently precipitating with addition of other components. A sol-gel type process was patented (U.S. Pat. No. 5,637,258 June 1997) by Goldburt et al for producing rare-earth activated metal oxide nanomaterials. They employed n-butoxide solution of host and long gelation procedure to prepare the specific category of product. U.S. Pat. No. 5,770,172 June 1998 by Linehan et al is similar to 1993 patent of Matson et al where nanosize of material prepared has been emphasised. Chhabra V et al filed another patent in (U.S. Pat. No. 6,036,886 March 2000) for preparation of activated i.e. doped metal oxide nanomaterials using microemulsion technique. In this two microemulsions containing compounds with individual radials constituting the oxide are separately prepared and mixed to form third emulsion with desired oxide. Glumac et al 1999 patented (U.S. Pat. No. 5,876,683) a method of combustion flame synthesis of nanophase materials. The method involves control thermal decomposition of one or more metalorganic precursors in a flat-flame combustor unit in which both temperature distribution and gas phase residence time are uniform over the entire surface of the burner.
Laine et al. patented in 1999 (U.S. Pat. No. 5,958,361) a process for preparation of ultra fine metal oxide particles by flame spray pyrolysis of solution containing gycolato polymetallooxanes dissolved in a volatile organic solvent. In another technique, patented by Yadav et al as per US Pat 2003138368 production of high purity nanomaterials is achieved by preparing liquid precursors of the desired nanomaterials first and then these are pumped into a plasma reactor maintained at 3000-6000° C. using DC arc and argon gas. For preparation of MgO aqueous solution of Mg acetate is used as precursor. A method and apparatus for direct electro thermal-physical conversion of ceramic into nanopowder has been patented vide patent U.S. Pat. No. 6,600,127 B1 July 2003 by Paterson et al. It utilizes a hybrid exploding wire device containing a solid metal wire fuse in a bore of a tube which is open at both ends. Bore is filled with ceramic powder whose nanophase is desired. An electrical discharge produces the plasma and hence the nano ceramic particles. Another method for producing semiconductor quantum particle was patented vide patent U.S. Pat. No. 6,623,559 B2 by Wen-Chiang Huang which uses super heated fine sized fluid droplets of a metal and a non-oxygen reacting element like p, As, S, Se and Te. U.S. Pat. No. 6,589,496 B1 describes synthesis of metal oxide doped cerium oxide by reacting aqueous cerium salt and metal ion solution with an alkali at temperature lower than 60° C. and at pH>5. Use of oxidizing agent is recommended. US patent application US 2004/0050207 describes gas phase synthesis of nanoparticles in a diffusion flame burner. Design of burner is important. Another patent EP 1378489 describes a method for production of metal oxides by flame spray pyrolysis employing high enthalpy solvents. U.S. Pat. No. 6,733,828 B2 filed by Chao et al in May 2004 on method of fabricating nanostructured materials describes confined synthesis of nanostructured material inside a mesoporous material. A monolayer of charged functional group of nanomaterial to be produced is attached on the inside wall of mesoporous material by reaction. Then nanomaterial is generated by reduction/oxidation etc. Mark and Gareth patented (GB2381530) a process for preparing water soluble particles of luminescent materials. It involves coating particles of the luminescent material with an organic acid or Lewis base such that the surface of the coating possesses one or more reactive group so that these can be used for biotagging. Jose et al claimed (EP1339075) synthesis of magnetic nanoparticles via decomposition of organometallic precursors in solution in presence of a reaction gas and a mixture of organic ligands. Another process for synthesis of nanomaterials particularly of carbon has been patented by Peter et al. (WO 2004007361). A patent US 2005/0087724 A1 relates to Preparation of Gadolinium-yttrium oxide with europium oxide and subsequent sintering under high pressure and temperature. Initial preparation is by combustion synthesis. There is no mention of control of valence state of any oxide. Another US patent application US 2005/0285083 describes a method of making luminescent nanomaterial of varying morphologies and sizes. Materials are lanthanide group phosphates with one or more lanthanides as dopants. After combustion reaction, the powders are heated in a controlled atmosphere at temperatures around 1000° C. Again no claim of lower valence state dopant or constituent is made.
Processes/techniques disclosed above in the prior art disclosures for preparation of nanomaterials involve use of large number of energy and cost intensive starting chemicals and generally there are many stages in the processes. None of the disclosures is describing nanomaterials in metal oxide form in lower valence state in shape of nanowires. These extra steps in the processes make the end product, a nanomaterial very costly. Some of the processes are for a specific type of chemical compound or an application.